1. Prior Art
The present invention relates to an organic positive temperature coefficient thermistor that is used as a temperature sensor or overcurrent-protecting element, and has PTC (positive temperature coefficient of resistivity) characteristics that its resistance value increases with increasing temperature.
2. Background Art
An organic positive temperature coefficient thermistor having conductive particles dispersed in a crystalline polymer has been well known in the art, as typically disclosed in U.S. Pat. Nos. 3,243,753 and 3,351,882. The increase in the resistance value is believed to be due to the expansion of the crystalline polymer upon melting, which in turn cleaves a current-carrying path formed by the conductive fine particles.
An organic positive temperature coefficient thermistor can be used as a self control heater, an overcurrent-protecting element, and a temperature sensor. Requirements for these are that the resistance value is sufficiently low at room temperature in a non-operating state, the rate of change between the room-temperature resistance value and the resistance value in operation is sufficiently large, and the resistance value change upon repetitive operations is reduced.
To meet such requirements, it has been proposed to incorporate a low-molecular organic compound such as wax in a polymer matrix. Such an organic positive temperature coefficient thermistor, for instance, includes a polyisobutylene/paraffin wax/carbon black system (F. Bueche, J. Appl. Phys., 44, 532, 1973), a styrene-butadiene rubber/paraffin wax/carbon black system (F. Bueche, J. Polymer Sci., 11, 1319, 1973), and a low-density polyethylene/paraffin wax/carbon black system (K. Ohe et al., Jpn. J. Appl. Phys., 10, 99, 1971). Self control heaters, current-limiting elements, etc. comprising an organic positive temperature coefficient thermistor using a low-molecular organic compound are also disclosed in JP-B's 62-16523, 7-109786 and 7-48396, and JP-A's 62-51184, 62-51185, 62-51186, 62-51187, 1-231284, 3-132001, 9-27383 and 9-69410. In these cases, the resistance value increase is believed to be due to the melting of the low-molecular organic compound.
One of advantages to the use of the low-molecular organic compound is that there is a sharp rise in the resistance increase with increasing temperature because the low-molecular organic compound is generally higher in crystallinity than a polymer. A polymer, because of being easily put into an over-cooled state, shows a hysteresis where the temperature at which there is a resistance decrease with decreasing temperature is usually lower than the temperature at which there is a resistance increase with increasing temperature. With the low-molecular organic compound it is then possible to keep this hysteresis small. By use of low-molecular organic compounds having different melting points, it is possible to easily control the temperature (operating temperature) at which there is a resistance increase. A polymer is susceptible to a melting point change depending on a difference in molecular weight and crystallinity, and its copolymerization with a comonomer, resulting in a variation in the crystal state. In this case, no sufficient PTC characteristics are often obtained. This is particularly true of the case where the operating temperature is set at less than 100.degree. C.
One of the above publications, Jpn. J. Appl. Phys., 10, 99, 1971 shows an example wherein the specific resistance value (.OMEGA.cm) increases by a factor of 10.sup.8. However, the specific resistance value at room temperature is as high as 10.sup.4 .OMEGA.cm, and so is impractical for an overcurrent-protecting element or temperature sensor in particular. Other publications show resistance value (.OMEGA.) or specific resistance (.OMEGA.cm) increases in the range between 10 times or lower and 10.sup.4 times, with the room-temperature resistance being not fully decreased.
In many cases, carbon black, and graphite have been used as conductive particles in prior art organic positive temperature coefficient thermistors including the above-mentioned ones. A problem with carbon black is, however, that when an increased amount of carbon black is used to lower the initial resistance value, no sufficient rate of resistance change is obtainable; no reasonable tradeoff between low initial resistance and a large rate of resistance change is obtainable. Sometimes, particles of generally available metals are used as conductive particles. In this case, too, it is difficult to arrive at a sensible tradeoff between low initial resistance and a large rate of resistance change.
One approach to solving this problem is disclosed in JP-A 5-47503 that teaches the use of conductive particles having spiky protuberances. More specifically, it is disclosed that polyvinylidene fluoride is used as a crystalline polymer and spiky nickel powders are used as conductive particles having spiky protuberances. U.S. Pat. No. 5,378,407, too, discloses a thermistor comprising filamentary nickel having spiky protuberances, and a polyolefin, olefinic copolymer or fluoropolymer.
However, these thermistors are still insufficient in terms of hysteresis and so are unsuitable for applications such as temperature sensors, although the effect on the tradeoff between low initial resistance and a large resistance change is improved. In addition, these thermistors have an operating temperature of 100.degree. C. or higher. Although some thermistors have an operating temperature in the range of 60 to 90.degree. C., they are impractical because their performance becomes unstable upon repetitive operations. When thermistors are used as protective elements for secondary batteries, electric blankets, heaters for lavatory seats and vehicle seats, etc., an operating temperature of 100.degree. C. or higher poses a great danger to the human body. With the safety of the human body in mind, the operating temperature must be below 100.degree. C. In recent years, organic positive temperature coefficient thermistors have been increasingly demanded as over-current protecting elements for portable telephones, personal computers, etc. In view of the temperature of 40 to 90.degree. C. at which they are usually used, too, thermistors having an operating temperature from 40.degree. C. to lower than 100.degree. C. are desired.
Thus, never until now is an organic positive temperature coefficient thermistor accomplished, which can show good performance at an operating temperature of less than 100.degree. C. and have high performance stability.
In Japanese Patent Application No. 9-350108, the inventors have already come up with an organic positive temperature coefficient thermistor comprising a thermoplastic polymer matrix, a low-molecular organic compound and a conductive particle having spiky protuberances. This thermistor has a sufficiently low room-temperature specific resistance of 8.times.10.sup.-2 .OMEGA.cm, a rate of resistance change of ten orders of magnitude greater between an operating state and a non-operating state, and a reduced temperature vs. resistance curve hysteresis. In addition, the operating temperature is equal to or greater than 40.degree. C. and less than 100.degree. C.
However, this thermistor is found to be insufficient in terms of performance stability, with a noticeably increased resistance at high temperature and humidity in particular. This appears to be due to the segregation, etc. of the working or active substance, i.e., the low-molecular organic compound upon repetitive melting/solidification cycles during operation, which segregation is ascribable to the low melting point and low melt viscosity of the low-molecular organic compound. This in turn causes a change in the dispersion state of the low-molecular organic compound and conductive particles, resulting in a performance drop. Such a performance stability problem is important to the low-molecular organic compound serving as the active substance.